1. Field of the Invention
The present invention relates to a braking mechanism for a castor, and more particularly to a braking device that locks up the wheels to a skid platform accurately and stably.
2. Prior Art
In accordance with the common design of an office chair, a skid platform and serving trolley, they all have rotatable castors attached to their legs. Some of the prior art castors are equipped with a braking mechanism that is able to stop the castor from rolling. As shown in FIG. 4 and FIG. 5, the braking mechanism of a conventional castor consists of a wheel support A having an axle suspension A1 with a housing A11, a pair of slotted holes A12 formed through opposing sides of the housing A11 and two skid shoes A2 extending outwardly from the top portion of the housing A11. A spring B and a slide C are sequentially seated in the housing A11. The slide C has a hole C1 in alignment with the slotted holes A12 for receiving the axle D therethrough. The two axial-tag terminals of the axle D are respectively plugged into two opposing dishing wheels E. In a first state of the wheels E, as shown in FIG. 4, the wheels are sitting on the ground. The wheels E, the axle D and the slide C work as a linked assembly, while the spring B on the slide C pushes up the wheel support A so that the axle D slides down along the slotted holes A12 of the axle suspension A1, until the skid shoes A2 at the top of the housing A11 touch against the inside walls of the wheels E. The skid shoes A2 function as a brake for the wheels E. When the wheel support A is pressed down by a force as shown in FIG. 5, the spring B in the axle suspension A1 is depressed downwardly so that the skid shoes A2 at the top of the housing A11 depart from contact with the wheels E, thereby releasing the wheels E so that they are free to roll.
Accordingly the design of the conventional castor brake depends only on the frictional force between the skid shoes and the inside wall of the wheels to stop movement of the trolley. That arrangement makes it difficult to completely stop the wheels. The wheels still have a differential motion with respect to the wheel support, and after a certain period of time, the skid shoes are worn where they contact the wheels, which degrades their braking effect.